Processes to separate alkane or alkene mixtures into their linear and branched component portions are an important necessity in industry. These separation processes generally use a solid adsorbent to selectively adsorb desired components from the mixture. The components are later desorbed and recovered. Typically the solid adsorbents used in industry are zeolites or other molecular sieve materials, and a wide variety of such adsorbents have been disclosed in the prior art. For example, the preferential adsorption of linear alkenes over branched alkenes using adsorbents such as ZSM-5 and silicalite was disclosed in U.S. Pat. Nos. 4,309,281 and 4,061,724, respectively. The crystalline molecular sieve silicalite was also used in U.S. Pat. No. 4,486,618 to separate linear alkenes having six carbon atoms from branched and cyclic alkenes having six carbon atoms and in U.S. Pat. No. 4,455,444 to perform the similar separation of linear alkanes having six carbon atoms from branched and cyclic alkanes having six carbon atoms. U.S. Pat. No. 4,992,618 disclosed the use of the commercially available type 5A molecular sieve to separate linear alkanes from non-linear alkanes.
The present invention expands the range of useful solid adsorbents to effect the separation of hydrocarbons according to whether components are branched or linear to include alkylene-bridged polysilsesquioxanes. Specific organically-bridged polysilsesquioxane materials and their preparation have been disclosed in Shea, K. J., Loy, D. A. Chem. Mater. 1989, 1, pp. 572-574; Shea, K. J., Loy, D. A., Webster, O. J. Am. Chem. Soc. 1992, 114, pp. 6700-6710; Shea, K. J., Loy, D. A., Webster, O. Mater. Res. Soc. Symp. Proc. 1990, 180, pp. 975-980; Shea, K. J., Loy, D. A., Webster, O. Polym. Mater. Sci. Eng. 1990, 63, pp. 281-285. Specific alkylene-bridged polysilsesquioxanes and their preparation have been disclosed in Oviatt, H. W., Shea, K. J., Small, J. H. Chem. Mater. 1993, 5, pp. 943-950 and Small, J. H., Shea, K. J., Loy, D. A. J. Non-Crystalline Solids 1993, 160, pp. 234-246. This art teaches that organic groups can be introduced at regular intervals in an inorganic silicate framework, thus forming a three-dimensional organic-inorganic hybrid silicate-like polymeric material, also called an organically-bridged polysilsesquioxane. A two-dimensional representation of the well-known inorganic silicate framework is shown in I, and an analogous representation of the organically-bridged polysilsesquioxane where the represents the organic bridging group is shown in II. Of course, frameworks I and II, in reality, extend to form a three-dimensional, continuous, amorphous solid. ##STR1##
The specific organic bridging groups disclosed in the art include ethylene, tetramethylene, hexamethylene, octamethylene, decamethylene, and tetradecamethylene. In view of the prior art, alkylene-bridging groups of trimethylene, pentamethylene, heptamethylene, nonamethylene, undecamethylene, dodecamethylene, tridecamethylene and their preparation are readily apparent to those skilled in the art. Organically-bridged polysilsesquioxanes containing these bridging groups are termed alkylene-bridged polysilsesquioxanes. One stated objective of the disclosed work was to provide molecular level control of the morphology of the framework, another was to provide a new chromatographic support, and a third was use in optical applications. However, applicant has found that these materials perform as adsorbents for linear and branched alkanes, alkenes, and alkynes at low temperatures. Furthermore, applicant has discovered that the adsorbed branched hydrocarbons may be desorbed from the alkylene-bridged polysilsesquioxanes using environmentally preferred alkane desorbents.